This invention relates generally to cooking appliances, and more particularly, to a solid-state control circuit for selectively operating a cooking appliance such as a deep fat fryer in any one of several modes, including a substantially constant temperature cooking mode, a low temperature "hold" mode during periods of non-use, a "melt" mode during which solidified fat is melted, and a "test" mode during which the safety thermostat and the safety contactor are tested.
Restaurants, institutions and other food services typically use deep fat fryers for preparing fried foods such as French fried potatoes, seafood, chicken and the like. To insure that the food is fried uniformly and to the desired crispness without becoming greasy, it is important to maintain the temperature of the cooking fat constant during the cooking operation in the range between 275.degree. Farenheit and 400.degree. Farenheit. Accordingly, prior devices for deep fat frying food have commonly included an operating thermostat for sensing the temperature of the cooking fat and for opening and closing an operating contactor interposed in series with the heating elements to maintain the fat at the desired frying temperature.
Other apparatus has been suggested for performing functions which are generally related to those of the present invention, but which are substantially different. These include, for example, complex electronic timing circuits for alerting an operator that the food has been fully cooked, such as that shown in U.S. Pat. No. 3,995,067. There a thermistor comprising one leg of a resistance bridge is mounted in a probe immersed in the cooking fat. The bridge is coupled across a comparator which senses the output voltage from the bridge and generates an output signal varying with temperature. This temperature related output signal is then compared in a second comparator with the voltage across a charging capacitor in a resistance-capacitance network having a charging time constant corresponding to the elapsed time from the beginning of the frying cycle. Thus, for instance, the length of cooking time cycle will be increased when the food is cooked at a lower temperature and as the temperature is increased, the time of the cooking cycle will decrease. When the charging voltage from the resistance-capacitance network exceeds the temperature-related output signal from the first comparator, the second comparator generates a signal to alert an operator that the frying cycle is completed, and the operator must then remove the food from the fat.
Other arrangements have also been developed for use in connection with conventional ovens and microwave ovens. For example, U.S. Pat. No. 4,035,787 discloses apparatus including a temperature sensing thermistor in a probe adapted for insertion into the food being cooked in an oven. The apparatus uses a plurality of voltage divider networks for providing reference input and operating temperature input information to a comparator, the thermistor being connected in one of the voltage divider networks to control the condition of the comparator to interrupt the supply of power to the heating element upon completion of the cooking cycle. This particular apparatus, however, is not suitable for cycling the heating elements on and off to maintain a constant temperature. Rather, the temperature is maintained at a constant level by an operating thermostat.
Not only is it important to maintain a constant cooking temperature during the cooking operation, it is also advantageous for several reasons to be able to reduce the heat applied to the cooking fat and thereby decrease the time during which the heating elements need be energized during periods of non-usage. First, if the fat is maintained at high cooking temperatures throughout the day, even when the fryer is not being used, the fat deteriorates and must be replaced more frequently. If, on the other hand, the fat is allowed to cool to a lower temperature during non-peak periods of usage, the useful life of the fat is increased. Further, maintaining the cooking fat at the higher cooking temperature throughout the day requires more energy than if the fat is allowed to cool to a lower temperature level during non-usage. However, the fat must be maintained at some minimum temperature, for example, 200.degree. Farenheit, so that the cooking fat can be reheated to the cooking temperature in a short time and does not solidify.
One such apparatus for maintaining the cooking fat at a lower temperature for an extended period of time is disclosed in U.S. Pat. No. 3,977,390 wherein the apparatus is manually switched to a standby condition by means of a toggle switch. It is desirable, however, to also provide means for automatically switching the appliance from the cooking mode to the lower temperature level mode after a predetermined time interval.
Another desirable feature is to provide means for adapting the fryer to melt solidified fat such as at the beginning of the day or whenever the fat is changed. When solidified fat is being melted, it is essential that the fat be heated slowly as it is melted. Rapid heating at high temperatures may cause the fat to burn and may overheat and damage those portions of the fryer which are heated but not immediately covered with melted fat, requiring replacement or costly repair.
Several prior devices have incorporated means for melting fat at a lower temperature. One such device which has been suggested is shown in U.S. Pat. No. 3,800,779 wherein the fry pot is provided with a melt thermostat which is responsive to the presence of a warm liquid shortening at a predetermined level in the container. The melt thermostat initially actuates a timer motor which functions to cylically actuate a bypass switch, thereby cyclically enabling and disabling a gas burner or an electric heating element. As soon as the fat in the fry pot has melted, the warm liquid shortening is detected by the melt thermostat which then functions to disable the timer motor. Thereafter, the regulation of the heat input to the fry pot is automatically transferred to conventional devices such as a high limit thermostat and an operating thermostat.
In the apparatus shown in U.S. Pat. No. 3,877,359, a melt cycle is provided by initially connecting the heating elements in series for a low heat melt cycle so that the fat is not burned during the start up period. The elements are then switched into a parallel arrangement for high heat cooking. The switch over from the low heat to the high heat modes can be manually actuated or can be temperature actuated, responsive to a thermostat, at a temperature just above the melting temperature of the fat.
Finally, although the optimum frying temperature is in the range between 275.degree. Farenheit and 400.degree. Farenheit, a malfunction in the circuitry of a deep fat fryer may result in the heating elements remaining on continuously to heat the cooking fat to temperatures in excess of 460.degree. Farenheit to a point near the flash point of the fat. Prior deep fat fryers have commonly included a safety thermostat to open the contacts of a safety contactor responsive to the detection of an overheat condition. The safety thermostat opens the safety contactor contacts at a predetermined temperature above the normal cooking temperature to deenergize the heating elements and prevent the fat from reaching the flash point, i.e., 475.degree. Farenheit, and igniting to cause a fire. In many prior art units, there is no means for routinely testing the safety thermostat or the safety contactor to verify that they are operable. Thus, if, for any reason, the safety thermostat or the safety contactor does become inoperative, it will not be discovered until there is a fire. Therefore, it is desirable to provide means for routinely checking the safety thermostat and the safety contactors to determine whether it is operational.